I have been thinking about this and I was wondering that if you work out including your pecs and someone goes and hits you in the pec, does that mean that they will bust it or something? This might be idiotic, but I am curious. What about stretching it or pressing it down?
The only way getting your pecs bigger could make it prone to bursting is if you did so by implanting a balloon. lol. But yeah, it's not like you are making your pectoral muscles fill with air when you work them out. You are building more muscle. It doesn't really hurt more the bigger they are. On the contrary, you don't feel a punch to the chest as much. When you don't have it built up, getting punched in the chest puts more impact on the bone, which hurts a lot more.
The reason your pecs get big when working out is because there is blood in those area repairing the torn muscle fibers. But nothing is able to burst. It would be no different than somebody hitting you when you haven't worked out.
Damage to your muscles by getting punched is not more likely if they are larger than if they are smaller. The muscle fibers are just as tough. The reason you can handle more stress with larger muscles is because there are more muscle fibers to distribute the work in larger muscles. But as far as impact, the muscles are in no more risk. Although, the larger muscles you have in your chest, the more protection your ribs have from damage, as well as the organs they cover.